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    SubjectRe: [rfc 2/3] fs, proc: Introduce the Children: line in /proc/<pid>/status
    On Thu, 1 Dec 2011 13:54:34 +0400
    Cyrill Gorcunov <gorcunov@gmail.com> wrote:

    > On Wed, Nov 30, 2011 at 10:05:37AM +0400, Cyrill Gorcunov wrote:
    > > On Wed, Nov 30, 2011 at 02:00:09PM +0900, KAMEZAWA Hiroyuki wrote:
    > > > On Tue, 29 Nov 2011 23:12:54 +0400
    > > > Cyrill Gorcunov <gorcunov@openvz.org> wrote:
    > > >
    > > > > From: Pavel Emelyanov <xemul@parallels.com>
    > > > >
    > > > > There is no easy way to make a reverse parent->children chain
    > > > > from the task status, in turn children->parent provided with "PPid"
    > > > > field.
    > > > >
    > > > > So instead of walking over all pids in system to figure out what
    > > > > children the task have -- we add explicit "Children" member to
    > > > > /proc/<pid>/status since kernel already knows this kind of information
    > > > > but it was not yet exported.
    > > > >
    > > > > Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
    > > > > Acked-by: Serge Hallyn <serge.hallyn@canonical.com>
    > > > > Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
    > > >
    > > > I may be too pessimistic but what amount of overhead will this add to
    > > > ps -elf/ top ? Assuming an environment 'ps -elf' is called once per a sec,
    > > > if there are 2000 processes, task_list lock is taken 2000 times by this patch.
    > > >
    > >
    > > Hi Kame, good point! Yes, it introduces latency on high loaded systems.
    > > I must admit I tested this patch on a regular system, where not that much
    > > processes were launched but technically I think more correct would be to
    > > switch to children file. I'll tune up the patch. Thanks!
    > >
    > > > Isn't it better to add /proc/<pid>/children file or dir (as task)?
    > > >
    > > > Thanks,
    > > > -Kame
    > > >
    >
    > What about this one?
    > ---
    > fs, proc: Introduce the /proc/<pid>/children entry
    >
    > There is no easy way to make a reverse parent->children chain
    > from the task status, in turn children->parent provided with "PPid"
    > field.
    >
    > So instead of walking over all pids in system to figure out what
    > children the task have -- we add explicit /proc/<pid>/children entry,
    > since kernel already knows this kind of information but it was not
    > yet exported.
    >
    > Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
    > Cc: Pavel Emelyanov <xemul@parallels.com>
    > Cc: Serge Hallyn <serge.hallyn@canonical.com>
    > Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>

    Yes, I like /children file. other points seems to be pointed out by other
    reviewers.

    Thanks,
    -Kame



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